In order to drive an AC motor by means of a direct-current power source, a drive method using an inverter is commonly employed. Typically, the inverter is subjected to switching control by an inverter drive circuit, and, for example, a pulse width modulation (PWM) waveform voltage is applied to the AC motor.
There has been known a method for supplying power to an AC motor by selectively switching, by means of switches, among PWM current control, mode, overmodulation control mode, and rectangular wave voltage phase control mode. When the voltage amplitude exceeds the peak value of a reference triangle wave, the control mode is switched from the PWM current control mode to the overmodulation control mode. Further, when the voltage amplitude exceeds a value obtained by multiplying the peak value of the reference triangle wave by 1.27, the control mode is switched from the overmodulation control mode to the rectangular wave voltage phase control mode. On the other hand, when the absolute value of the actual current phase becomes less than the absolute value of the current command phase, the control mode is switched from the rectangular wave voltage phase control mode to the overmodulation control mode. When the voltage amplitude becomes less than the peak value of the reference triangle wave, the control mode is switched from the overmodulation control mode to the PWM current control mode.
According to the above method, in order to eliminate the influences of periodic noise and higher harmonic waves included in the d-axis current and the q-axis current, a value obtained by performing low-pass filter processing with respect to a measured current value is used as the pseudo actual current value for calculating the actual current phase. However, in cases in which the low-pass filter processing is performed, an undesirable situation is caused at a point when the torque command value is suddenly reduced. That is, during comparison of the absolute value of the current command phase and the absolute value of the actual current phase, the actual current phase calculated based on the pseudo actual current value subjected to the filter processing (shown by dotted line A in FIG. 11) becomes significantly deviated from the real actual current phase obtained from the original actual current (shown by bold line B in FIG. 11), as illustrated by the Lissajous curves of d-axis current and q-axis current shown in FIG. 11. As a result, when the switching from the rectangular wave voltage phase control mode to the overmodulation control mode is performed at the point (point a in FIG. 11) when the absolute value of the actual current phase calculated based on the current subjected to the filter processing becomes smaller than the absolute value of the current command phase (shown by thin line C in FIG. 11), the switching of the control mode from the rectangular wave voltage phase control mode to the overmodulation control mode is in fact effected at the point (point b in FIG. 11) when the actual current phase is deviated from the current command phase by an amount corresponding to the time constant of the low-pass filter. This type of delay in the switching of control modes generates undesirable hunting (vibrations) or the like in the current phases, possibly resulting in unstable control.